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Forsa Industries Rear Entry Sheet Metal Manifold
Runner 1 329 CFM
Runner 2 326 CFM
Runner 3 314 CFM
Runner 4 295 CFM
Average 316 CFM
Runner to runner max variation 34 CFM or 10.3%
Notes: One piece runner (ie: no TGV). If there was a radius or pipe
on the throttle body flange, the number 4 runner wouldn't
lag behind the others in terms of flow; I tested this by
simply holding my index finger next to & slightly below
the throttle flange opening which raised this runner's
flow to 317 CFM. The close proximity of this runner to
the open throttle flange makes it a sharp bend.

Process West Holy Moly Thing of Beauty
Runner 1 349 CFM
Runner 2 356 CFM
Runner 3 360 CFM
Runner 4 374 CFM
Average 359.75 CFM
Runner to runner max variation 25 CFM or 6.9%
Notes: Large plenum and short runner characteristics
cannot be seen in this test but my hypothesis is that
this could be a candidate for best choice in a big
turbo/big rpm setup.

Thanks to Tyler Williams the rainbow dude & Will @ Elevated Tuning and for their help and to Brian L (RexFTW) for the Process West, Zach (Fate066), Fin @ Forsa and Jordan (MartinSTi05) for the manifolds to test.

I really appreciate you posting these test results. I have always been up in the air about switching to a oem sti manifold or 2.5rs mani. Now I'm just curious on what my ported 03 wrx manifold will flow to see if it would be worth it.

I have had my hands on plenty wrx and sti intake manifold and never really saw any differance besides tb bolt pattern and color.

^This is what I'll be running.
Finally some numbers on a flow bench. The only other manifolds I'd like to see is the Cosworth and the vers 9 jdm long runner which would probably have the same results as any stock sti manifold.

Max, all of the big plenum SOHC are from newer EJ253s (think 06+), exact years and models I don't know, possibly someone (hey Zach) could chime in with more exact info.

We did this because all we ever heard was speculation without any real data, we didn't expect the big plenum NA manifolds to be so exceptional. We did hypothesize that they would have great runner to runner balance but we had figured they would only marginally outflow the STi and get destroyed by the SMIM. Testing revealed that the SMIM has poor balance, front runners (1&3) vs. back runners (2&4).
I am excited to test a Cossie as this will be great reference point when considering Cosworth's own flow numbers on the stocker when corrected from 50" (421 avg cfm) to 28"(315 avg cfm which is still 12% higher than our findings), the cast Cossie should make 350+ avg cfm on our bench corrected to 28" yet I am guessing somewhere closer to 315 avg cfm.

Excellent work. This is one thing this site and the Subaru community has been lacking for a LONG time.

It's interesting to see the gross variation of the e-Bay cheapy.

I'm curious, there are quite a few other manifolds out there. Are you guys willing to test more manifolds if people sent them in to you? I'm just thinking of things like old cast versus new plastic NA manifolds, the very large diameter runner Legacy (GT?) manifold, the Cosworth high flowmanifold, even some stock manifolds with some porting work (to see if this is worth it, although may be more beneficial attacked to a head and pulled though versus the manifold itself).

I would just love to see more comprehensive data for a lot of the major products out there.

I support a hard numbers style of testing. Many loud mouths are quick to condemn any product they don't have any real world experience with. Many think the cissies manifold kills low end etc etc. I sure had a healthy power curve with it, oh wait that's because the rest of my setup was thought out.

This type of testing eliminates many variables that cloud other conclusions people have made. That being said it would be nice to see dynamic testing that could quantify the effects of the runner length, ID, firing order and plenum volume. Maybe a "standard" test Subaru setup...

Here's some additional CFM numbers for you guys. I had the OEM JDM V7 EJ207 STi Intake Manifold & TGV's sent out to Extrude Hone and they threw it up on the flow bench before and after the Extrude Hone process. Impressive CFM gains in my opinion considering the expensive aftermarket options that are available.

I support a hard numbers style of testing. Many loud mouths are quick to condemn any product they don't have any real world experience with. Many think the cissies manifold kills low end etc etc. I sure had a healthy power curve with it, oh wait that's because the rest of my setup was thought out.

This type of testing eliminates many variables that cloud other conclusions people have made. That being said it would be nice to see dynamic testing that could quantify the effects of the runner length, ID, firing order and plenum volume. Maybe a "standard" test Subaru setup...

I just wish I had been able to get a flowbench that I was gonna get last fall. It was swiped from under my feet 1 day before I could pick it up

I'd love to see what someone with access to CFD & wet flow could come up with.

That said, we're not super interested in testing someone's home ported stock WRX manifold and I wouldn't be super excited about even a JDM long runner (I'd probably still do one) as I think we'd find that it's awfully close to a USDM STI.

I have a plastic Leggy XT manifold somewhere that I wanna do and I'd like to maybe test a Cossie, Forsa, Corsa-Veloce, Tomei and maybe a Tomei knockoff.

I support a hard numbers style of testing. Many loud mouths are quick to condemn any product they don't have any real world experience with. Many think the cissies manifold kills low end etc etc. I sure had a healthy power curve with it, oh wait that's because the rest of my setup was thought out.

This type of testing eliminates many variables that cloud other conclusions people have made. That being said it would be nice to see dynamic testing that could quantify the effects of the runner length, ID, firing order and plenum volume. Maybe a "standard" test Subaru setup...

I had ideas about building a throttle body flange adapter and flipping the switch on the machine to blow through it but there were two major issues with that plan: 1, no one else's numbers have been done that way and 2, we'd surely run out of capacity on our little machine. I thought it may be a better indicator on a forced induction setup but alas we stuck with the standard procedure.

^ I agree, but I really haven't seen a differance between sti and wrx manifolds. The test was performed with tgv deletes and sti mani so id assume a wrx manifold will be very close. I'm sure grimmspeed has some back up flow data from ported to non ported we can look at.

I had ideas about building a throttle body flange adapter and flipping the switch on the machine to blow through it but there were two major issues with that plan: 1, no one else's numbers have been done that way and 2, we'd surely run out of capacity on our little machine. I thought it may be a better indicator on a forced induction setup but alas we stuck with the standard procedure.

Unforunately we don't gave access to all the toys we can dream of as a community. However this is one hell of a good start.